First I downloaded Oracle Developer Tools for Visual Studio 2015 from here. In my reference list I added Oracle.ManagedDataAccess. My App.config looks like:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <connectionStrings>
    <add name="maxaDB"
      connectionString="User ID=userName;Password=pass;Data Source=(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=ipAddress)(PORT=myPort))(CONNECT_DATA=(SERVER=dedicated)(SERVICE_NAME=XE)))"
    />
  </connectionStrings>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
</configuration>

User ID and Password have to go before Data Source. Then I added "Oracle.ManagedDataAccess.Client;" in using list, and after that connecting was simple:

string maxaDb = ConfigurationManager.ConnectionStrings["maxaDB"].ConnectionString;
OracleConnection conn = new OracleConnection();